Japan's Prime Minister Shinzo Abe speaks at a meeting at his official residence in Tokyo January 21, 2015. [Photo/Agencies]

In past international hostage situations involving Japanese captives, at least one has been killed while most have been released. It is unclear how many times Japan paid ransom. The only confirmed case was in Kyrgyzstan in 1999.

Japanese media reported Wednesday, citing unnamed officials, that Goto's wife received an email on her mobile phone in December demanding a ransom of more than 2 billion yen ($17 million).

She exchanged several emails with the unknown person, whose email address was similar to one used by the Islamic State group, the Kyodo News service and other media reported. The messages did not include a threat to kill Goto, it said.

Securing the hostages' release will be hard, "because all Japanese diplomats left Syria as the civil war escalated," Tsutomu Ishiai, foreign news editor for the major newspaper Asahi Shimbun, said in a commentary.

"Japan is in an extremely difficult situation. It needs to find a way to save the hostages by getting in touch with religious leaders and local heavyweights who are in a position to make contact with the extremists," he added.

Shuji Hosaka, a senior research fellow at Japan's Institute of Energy Economics in Tokyo, said he did not believe the Islamic State group was specifically targeting Japanese when Goto and Yukawa were captured.

The few Japanese taken hostage by extremists so far generally have been caught up coincidentally in ambushes and attacks.

But the ransom demand appears to have been timed to coincide with Abe's visit, he said.

"I think they were waiting for the best time to release the video and information about them," Hosaka said. "Then Abe happened to be making a visit to the Middle East and there was a reference to the Islamic State and Japanese government aid during the visit. So they may have seen this as a good time to use these two Japanese as negotiation cards."